Carbon black for tire tread rubber

ABSTRACT

There is disclosed carbon black for a tire tread rubber which has a nitrogen adsorption specific surface area (N2SA) of 60 to 160 m2/g and a dibutyl phthalate absorption (DBP) of 90 to 150 ml/100 g which belong to the respective regions of hard grades of carbon black, and an intraaggregate void volume Vp (ml/g) which is at most equal to the value calculated according to the formula.

This application is a continuation of now pending application Ser. No.772,388, filed Oct. 7, 1991, now abandoned which application was acontinuation of Ser. No. 627,908, filed Dec. 17, 1990, now abandoned,which application was a continuation of Ser. No. 408,345, filed Sep. 18,1989, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to carbon black for a tire tread rubber,and more particularly, to carbon black which can remarkably lower theheat build-up in a tire tread rubber while keeping the abrasionresistance of the tire tread rubber on a conventional level.

The effect of reinforcing a rubber by carbon black has heretofore beenconsidered to be largely governed by the specific surface area (particlesize) and the structure of carbon black. Accordingly, there are knownmany grades of carbon black differing in these properties.

In compounding carbon black into a rubber component, an appropriatechoice is made of the grade of carbon black having characteristicsadapted to the application of a rubber composition to be prepared. Forexample, a hard grade of carbon black, such as N110 or N220, is used ina rubber member requiring a high abrasion resistance, such as a tiretread portion subject to severe running conditions. As the runningconditions for tires have recently become more and more severe, however,such a high performance has been required of a tire tread portion so asto satisfy particularly a high abrasion resistance and a low heatbuild-up at the same time.

In general, it is known that the abrasion resistance of a tire treadportion is enhanced as the specific surface area and structure of carbonblack compounded thereinto are increased. However, it is also known thatthe heat build-up in the tire tread portion is increased in keeping withincreases in the specific surface area and structure of carbon black.Thus, the abrasion resistance has an antinomic relation with the heatbuild-up. Accordingly, it has been considered extremely difficult tosimultaneously impart a high abrasion resistance and a low heat build-upto a rubber composition.

With a view to solving such a difficulty, there have been proposedvarious attempts to use carbon black having specified properties (see,e.g., Japanese Patent Publication No. 53-34149 and Japanese PatentApplication Kokai Publication No. 63-112638). Despite such proposals,however, no rubber compositions containing, compounded thereinto, carbonblack having such specified properties can simultaneously secure thesatisfactory levels of abrasion resistance and heat build-up, in whichfurther improvements has therefore been demanded.

In view of the above, the inventors of the present invention have madeinvestigations on the intraaggregate void volume of carbon black andfound that, when carbon black in the form of aggregates having a certainstructure level and a relatively small intraaggregate void volume iscompounded into a rubber component, the resulting rubber composition hasa high abrasion resistance and a low heat build-up. The presentinvention has been completed based on this finding.

SUMMARY OF THE INVENTION

A first object of the present invention is to provide carbon black whichcan remarkably lower the heat build-up in a tire tread rubber whilekeeping the abrasion resistance of the tire tread rubber on aconventional level.

A second object of the present invention is to provide carbon blackwhich is useful in a tread rubber of tires for passenger cars requiringparticularly a reduced fuel consumption.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of an example of a reactor tobe used in the production of the carbon black of the present invention;and

FIG. 2 is an enlarged cross-sectional view of the essential portion ofthe reactor shown in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The carbon black for a tire tread rubber according to the presentinvention has a nitrogen adsorption specific surface area (N₂ SA) of 60to 160 m² /g and a dibutyl phthalate absorption (DBP) of 90 to 150ml/100 g which belong to the respective regions of hard grades of carbonblack, and an intraaggregate void volume Vp (ml/g) which is at mostequal to the value calculated according to the formula:(0.00976×DBP)-0.1309. Among the above-mentioned characteristicproperties, the nitrogen adsorption specific surface area (N₂ SA) of 60to 160 m² /g and the dibutyl phthalate absorption of 90 to 150 ml/100 g,which are a particle-related range and a structure-related range,respectively, belonging to the respective regions of hard grades ofcarbon black, are prerequisites for the carbon black to impart a highabrasion resistance to a compounding rubber. When the nitrogenadsorption specific surface area (N₂ SA) is smaller than 60 m² /g, orwhen the dibutyl phthalate absorption is lower than 90 ml/100 g, carbonblack cannot impart a high abrasion resistance to a tire tread. When thenitrogen adsorption specific surface area (N₂ SA) exceeds 160 m² /g, thedispersibility of carbon black in a rubber component is reduced tothereby inhibit an improvement in the abrasion resistance thereof andbring about a high heat build-up. When the dibutyl phthalate absorptionexceeds 150 ml/100 g, carbon black unfavorably lowers the ice skidperformance of a tire tread.

The value calculated according to the formula: [0.00976×DBP-0.1309],which is considered to be the upper limit of the intraaggregate voidvolume Vp (ml/g) of the carbon black of the present invention, revealsthat the carbon black of the present invention has a smallintraaggregate void volume or, in other words, a low anisotropy ascompared with those of the equivalent grades of conventional carbonblack. When the intraaggregate void volume Vp (ml/g) is equal to orsmaller than the value calculated according to the above-mentionedformula, carbon black can remarkably lower the heat build-up in a tiretread while imparting thereto an abrasion resistance comparable to thoseof tire treads containing, compounded thereinto, the equivalent grade ofconventional carbon black.

In general, carbon black having a small intraaggregate void volume Vpessentially has an aggregate texture with a suppressively developedstructure, which works to lower the heat build-up in a tire tread rubberwhen the carbon black is compounded into the rubber. Particularly,carbon black satisfying the requirement of properties that theintraaggregate void volume Vp thereof should be at most equal to thevalue calculated according to the formula: (0.00976×DBP)-0.1309 has alow anisotropy, which can work to remarkably lower the heat build-up ina rubber component. The nitrogen adsorption specific surface area (N₂SA) of 60 to 160 m² /g and the dibutyl phthalate absorption of 90 to 150ml/100 g, which are prerequisites for carbon black to impart a highabrasion resistance to a rubber component as described above, may servesynergistically with the above-mentioned intraaggregate void volume Vpto remarkably lower the heat build-up in a rubber component whilesecuring a conventional level of abrasion resistance thereof.

The characteristic values of the carbon black of this invention weremeasured according to the following methods.

(1) Nitrogen adsorption specific surface area (N₂ SA):

ASTM D 3037-78 "Standard Methods of Testing Carbon Black - Surface Areaby Nitrogen Adsorption" Method B.

The N₂ SA of IRB No. 5 measured according to this method was 80.3 m² /g.(IRB stands for Industry Reference Black.)

(2) Dibutyl phthalate absorption (DBP):

JIS K6221 (1975) "Method of Testing Carbon Black for Rubber", Section6.1.2, Method A (corresponding to ASTM D2414-82)

A prescribed quantity of dry carbon black is placed in the mixingchamber of an absorptometer. Dibutyl phthalate is added dropwise to thecarbon black from a buret with mixing. The buret is closed automaticallyby the action of a limit switch when the torque of the rotor in themixing chamber reaches a predetermined value. The absorption iscalculated from the following equation: ##EQU1## wherein DBP: absorptionof dibutyl phthalate (ml/100 g)

V: volume of dibutyl phthalate added (ml)

W_(D) : quantity of dry carbon black (g)

(3) intraaggregate void volume Vp (ml/g)

Using a mercury porosimeter Poresizer 9300 manufactured byMicromeritics, a carbon black sample is immersed in mercury, to which aslowly increasing pressure is applied to gradually infiltrate themercury into the micropores of the carbon black in accordance with thepressure. The intraaggregate void volume is calculated from therelationship between the pressure and the amount of mercury infiltratedaccording to the equation (1). ##EQU2## wherein

X₁ : reading of the mercury porosimeter at 25 psi

X₂ : reading of the mercury porosimeter at 30,000 psi

W: weight of carbon black sample (g)

CF: constant determined by a cell used in the measurement.

Additionally stated, the intraaggregate void size corresponding to theapplied pressure of 25 psi is 7.2 μm, while that corresponding to theapplied pressure of 30,000 psi is 0.006 μm.

The carbon black of the present invention is produced using, forexample, a Y-shaped oil furnace as shown in FIG. 1 (see Japanese PatentPublication No. 52-10581). This oil furnace comprises two generators 1and 1' and a main reaction zone 2 extending from a position where thetwo generators converge. Each generator is made up of a wind box 4, aburner 5, a combustion chamber 7 having therein a feedstock oil spraynozzle 6, and a pyrolysis duct 8 integrated with the combustion chamber7. The hydrocarbon feedstock oil is sprayed into the combustion gas offuel oil via the feedstock oil spray nozzle 6 so that the oil spray ispyrolyzed to form a gas stream of carbon black intermediate. The two gasstreams of carbon black intermediate are simultaneously entrained intothe reaction chamber 2 at a high speed to impinge against each other atpoint P in a space 9. Thereafter, the resulting stream is cooled withwater sprayed at the position of a cooling water spray 3 and carbonblack is then separated therefrom. The conditions of forming the gasstreams of carbon black intermediate in the generators 1 and 1' arecontrolled to adjust the intraaggregate void volume Vp of the resultingcarbon black, while the conditions of burning in the furnace, theresidence time in the furnace of the stream of carbon black beingproduced, etc. are controlled to adjust the nitrogen adsorption specificsurface area (N₂ SA) and dibutyl phthalate absorption thereof. In theforegoing manner, carbon black having the characteristic propertiesspecified in the present invention can be produced.

The carbon black of the present invention is compounded into anelastomer, examples of which include a natural rubber, astyrene-butadiene rubber, a polybutadiene rubber, an isoprene rubber, abutyl rubber, and other various synthetic rubbers and mixed rubberscapable of being reinforced with common carbon black.

35 to 100 parts by weight of the carbon black of the present inventionis compounded into 100 parts by weight of a rubber component. The carbonblack and the rubber component are kneaded together with other necessarycomponents such as a vulcanizing agent, a vulcanization accelerator, anage resister, a vulcanization aid, a softener, and a plasticizer toprepare a rubber composition for tire treads.

As described above, the carbon black of the present invention has alow-anisotropy aggregate texture with a suppressively developedstructure, which can work to remarkably lower the heat build-up in arubber component while securing a conventional level of abrasionresistance thereof. Accordingly, the carbon black of the presentinvention can be suitably used in a tread rubber of tires especially forpassenger cars requiring a reduced fuel consumption.

Examples of the present invention will now be described in comparisonwith Comparative Examples.

The methods of measuring various characteristic properties of vulcanizedrubber compositions in Examples and Comparative Examples are as follows.

(a) Abrasion Loss

Abrasion loss was measured with a Lambourne abrasion tester (withmechanical slip mechanism) under the following conditions:

test piece: 10 mm thick, 44 mm in outside diameter

Emery wheel: GC type; grain size: #80; hardness: H

carborundum added: grain size: #80, adding rate: approximately 9 g/min

relative slip ratio of Emery wheel surface to test piece: 24%, 60%

speed of revolution of test piece: 535 rpm load on test piece: 4 kg

(b) Hysteresis Loss (tan δ)

Hysteresis loss was measured with a viscoelastic spectrometer(manufactured by Iwamoto Seisakusho Co.) under the following conditions:

test piece: 2 mm thick, 30 mm long, 5 mm wide

temperature: room temperature

frequency: 50 Hz

dynamic strain (amplitude): ±1%

(c) Other Properties

All other measurements were made according to JIS K6301 "Physical TestMethod for General Rubbers"

Example 1 Production of Carbon Black

The oil furnace used has a Y-shaped structure as shown in FIG. 1 whichcomprises two generators 1 and 1' so arranged as to converge at an angleof 60° with each other in front of a main reaction zone 2 having a frontnarrow portion 9 of 90 mm in inside diameter and 900 mm in length and arear broad portion 10 of 200 mm in inside diameter and 2,000 mm inlength. Each generator comprises a pyrolysis duct 8 (60 mm in insidediameter and 600 mm long) and a combustion chamber 7 (400 mm in insidediameter and 800 mm long, including 200 mm of conical sections) providedwith a burner 5 and a feedstock oil spray nozzle 6 arranged coaxiallywith each other through a wind box 4 provided around a front portionthereof. A ring member 11 having a constriction ratio of 0.90 isprovided 50 mm downstream of the intersectional point P in the frontnarrow portion 9 as shown in FIG. 2. The ring member 11 is made of afire brick. The constriction ratio m is expressed by the followingequation: ##EQU3##

wherein D=90 mm and D₀ =85 mm.

The feedstock oil used was an aromatic hydrocarbon oil having a specificgravity (15/4° C.) of 1.0703, an Engler viscosity (40/20° C.) of 2.10, abenzene-insolubles content of 0.03%, a correlation index (BMCI) of 140and an initial boiling point of 103° C. The fuel oil used was ahydrocarbon oil having a specific gravity (15.4° C.) of 0.903, aviscosity (at 50° C.) of 16.1 cSt, a residual carbon content of 5.4%, asulfur content of 1.8% and a flash point of 96° C.

Four kinds of carbon black (Runs Nos. 1 to 4) according to the presentinvention were produced from the above-mentioned feedstock oil using theabove-mentioned oil furnace and fuel oil under conditions as listed inthe following Table I.

                                      TABLE I                                     __________________________________________________________________________            Total air                                                                          Fuel oil                                                                            Fuel combus-                                                                         Feedstock oil                                                                        Residence                                    Run Gener-                                                                            feed rate                                                                          feed rate                                                                           tion rate                                                                            feed rate                                                                            time                                         No. ator                                                                              (Nm.sup.3 /H)                                                                      (kg/H)                                                                              (%)    (kg/H) (msec)                                       __________________________________________________________________________    1   1   200  10.3  180    63.1   8.3                                              1'  250  12.8  180    73.5                                                2   1   220  11.3  180    48.5   7.5                                              1'  280  14.4  180    53.5                                                3   1   260  14.1  170    46.0   6.4                                              1'  300  16.3  170    48.8                                                4   1   280  15.2  170    42.1   5.9                                              1'  330  17.9  170    44.8                                                __________________________________________________________________________

Table II shows the data on the four kinds of carbon black thus producedwith respect to nitrogen adsorption specific surface area (N₂ SA),dibutyl phthalate absorption, intraaggregate void volume Vp, and valuecalculated according to the formula: (0.00976×DBP)-0.1309.

"As noted from the characteristics of the carbon black of Example 1, theDBP value of 105 and the V_(p) value of 0.87 provide a narrower rangewhere the V_(p) is at most equal to the value calculated according tothe formula: (0.00976×DBP)-0.1548."

The data on Runs Nos. 5 and 8 in Table II which are listed ascomparative examples are those of hard grades of carbon black producedby conventional techniques, which have a nitrogen adsorption specificsurface area (N₂ SA) of at least 60 m² /g but an intraaggregate voidvolume Vp falling outside the range specified in the present invention.

                                      TABLE II                                    __________________________________________________________________________             Example     Compartive Example                                                Run No.                                                              Properties                                                                             1  2  3  4  5 (HAF)                                                                            6 (ISAF)                                                                           7 (SAF)                                                                            8                                         __________________________________________________________________________    N.sub.2 SA (m.sup.2 /g)                                                                 81                                                                              114                                                                              137                                                                              150                                                                               76  117  138  152                                       DBP (ml/100 g)                                                                         105                                                                              116                                                                              121                                                                              140                                                                              103  116  113  146                                       V.sub.p (ml/g)                                                                         0.87                                                                             0.95                                                                             1.02                                                                             1.14                                                                             0.95 1.07 1.04 1.36                                      value of equation                                                                      0.89                                                                             1.00                                                                             1.05                                                                             1.23                                                                             0.87 1.00 0.97 1.29                                      __________________________________________________________________________

Example 2

Each of the four kinds of carbon black produced in Example 1 was blendedwith natural rubber and other components at a blending ratio as shown inTable III.

                  TABLE III                                                       ______________________________________                                        Components         parts by weight                                            ______________________________________                                        natural rubber (RSS #1)                                                                          100                                                        carbon black       50                                                         aromatic oil (softener)                                                                          4                                                          stearic acid       3                                                          (dispersing vulcanization aid)                                                zinc oxide (vulcanization aid)                                                                   5                                                          dibenzothiazyl disulfide                                                                         1                                                          (vulcanization accelerator)                                                   sulfur (vulcanizing agent)                                                                       2.5                                                        ______________________________________                                    

The compound as shown in Table III was vulcanized at a temperature of145° C. for 40 minutes to prepare a rubber composition, which was thenexamined with respect to various rubber characteristics. The results areshown in Table IV with the same Runs Nos. as those of carbon black inTable II.

                                      TABLE IV                                    __________________________________________________________________________                 Example         Comparative Example                                           Run No.                                                          Properties   1   2   3   4   5   6   7   8                                    __________________________________________________________________________    Hardness (JIS Hs)                                                                           63  64  65  66  63  63  65  66                                  300% Modulus (kg/cm.sup.2)                                                                 111 119 124 131 110 119 113 133                                  Tensile strength (kg/cm.sup.2)                                                             273 283 288 290 271 284 287 291                                  Elongation (%)                                                                             580 575 550 535 580 570 600 530                                  Abrasion loss                                                                 slip ratio 24%                                                                             0.0874                                                                            0.0746                                                                            0.0699                                                                            0.0685                                                                            0.0891                                                                            0.0747                                                                            0.0711                                                                            0.0682                               slip ratio 60%                                                                             0.1201                                                                            0.1013                                                                            0.0948                                                                            0.0876                                                                            0.1211                                                                            0.1011                                                                            0.0970                                                                            0.0870                               Hysteresis (tan δ)                                                                   0.224                                                                             0.247                                                                             0.262                                                                             0.260                                                                             0.241                                                                             0.268                                                                             0.270                                                                             0.286                                __________________________________________________________________________

As is apparent from the results shown in Table IV, the rubbercompositions of Runs Nos. 1 to 4 into each of which carbon blackaccording to the present invention was compounded were remarkablylowered in hysteresis (tan δ) as an indicator of heat build-up whilesecuring substantially the same level of abrasion resistance as those ofthe rubber compositions of Runs Nos. 5 to 8 in Comparative Example intoeach of which the equivalent grade of conventional carbon black wascompounded.

What is claimed is:
 1. Carbon black for a tire tread rubber which has anitrogen adsorption specific surface area (N₂ SA) of 60 to 160 m² /9 anda dibutyl phthalate absorption (DBP) of 90 to 150 ml/100 g which belongto the respective regions of hard grades of carbon black, and anintraaggregate void volume V_(p) (ml/g) which is at most equal to thevalue calculated according to the formula:

    (0.00976×DBP)-0.1548.